Conventional mechanically driven valve trains operate the intake and exhaust valves based on the position and profile of lobes on a camshaft. The engine crankshaft is connected to the pistons by connecting rods and to the camshaft by a belt or chain. Therefore, the intake and exhaust valve opening and closing events are based-on the crankshaft position. This relationship between the crankshaft position, piston position, and valve opening and closing events determines the stroke of a given cylinder, e.g., for a four stroke engine the intake, compression, power, and exhaust strokes. As a result, engine starting and the first cylinder to fire are determined in part by the camshaft/crankshaft timing relationship and the engine stopping position.
On the other hand, electromechanically driven valve-trains do not have the physical constraints that tie the camshaft and crankshaft together, i.e., there may not be belts or chains linking the camshaft and crankshaft, at least for some valves. Furthermore, full or partial electromechanical valve-trains may not require a camshaft. Consequently, the physical constraints linking the camshaft and crankshafts are broken. As a result, additional flexibility to control valve timing is possible when electromechanical valves are used in an internal combustion engine.
One method to control electromechanical valve operation during an engine operation is described in U.S. Pat. No. 5,765,514. This method provides for an injection sequence for the cylinders that is initialized when a first crankshaft pulse is generated after generation of a first signal pulse representing crankshaft rotation through 720 degrees. The injection sequence and crankshaft position sequence correspond to the position of each cylinder, whereby the opening/closing timing of each intake valve and exhaust valve can be controlled. The cylinders are set to the exhaust stroke, suction stroke, compression stroke, and explosion stroke, respectively.
Once the above-mentioned method has set the stroke of each cylinder, timing of valve opening and closing is determined by retrieving a map based on accelerator pedal position and engine speed. Finally, a further valve timing adjustment is made to correct for air-fuel errors. In general, during a start, accelerator pedal position remains constant and in a low demand position while the oxygen sensor is not available due to low sensor temperature. As a consequence, engine speed is the primary input used to determine valve timing during a start.
The above-mentioned method can have several disadvantages. Namely, valve timing that is based primarily on engine speed can result in further variation of engine speed, since changes in valve timing can affect engine speed. Also, it can create cylinder air-fuel ratio errors that may lead to increased emissions that result from inaccuracy of engine speed calculations. For example, calculating instantaneous engine speed during engine starting can produce errors in the determined speed that are introduced by sample frequency, calculation method, and sensor signal to noise ratio due to engine acceleration. Any deviation or variation of the determined engine speed from actual engine speed can thus result in an inadvertent and potentially unnecessary valve timing adjustment. Such adjustments can thus result in sub-optimal timing and performance.